xResonance is a sympathetic string simulator. It's based on a physical model of 14 independently tunable strings with some bells and whistles added. Well, it doesn't really have any bells and whistles — just strings. It does have a 5 band parametric EQ and some LFO's to help shape the sound just the way you want it, though. Download it and give it a spin — or some vibrations. It is useful to read the brief manual to get an idea of how to tweak the physical parameters to get the sympathetic strings to respond they way you want.

xResonance is a great tool for emulating instruments that have sympathetic strings or adding ambience to your instrument. It's also useful for intonation practice.